A nanotube is a material which has a long and thin cylindrical shape with a diameter of nanometer scale. According to the wall structure, nanotubes can be categorized into single-walled nanotubes (SWNT), in which the wall consists of one single layer, and multi-walled nanotubes (MWNT), in which the wall consists of multiple layers. Typically, the diameter of SWNT is not more than 1 nm and the diameter of MWNT is in the range of 10-100 nm; however, their diameters may be varied depending on the production method and conditions used. Although nanotubes are generally known to be a few micrometers in length, studies which disclose the development of nanotubes having a length of up to several millimeters have been recently reported.
As one of the representative examples of nanotubes, carbon nanotubes (CNT) can be mentioned by way of specific example. CNT are carbon allotropes containing carbon, which is one of the materials present in the earth in large quantities. CNT have a long and thin generally tubular shape in which carbon atoms are bound to each other in a network forming a hexagonal pattern that somewhat resembles a honeycomb. CNT have a number of advantageous properties, such as being almost one hundred times as strong as steel with a weight as light as aluminum; having a superior ability to deliver electricity as compared to copper; having a strong resistance to chemical and physical environments; being able to bind large quantities of various other chemical substances due to their tube-type shape which provides a relatively large surface area; possessing semiconducting or metallic properties; and other recognized advantages. Owing to such various advantages, CNT have been widely studied and utilized in a wide variety of fields including as electron emitters in various devices, vacuum fluorescent displays (VFD), white light sources, field emission displays (FED), electrodes for lithium secondary batteries, hydrogen-storage fuel cells, nano-wires, nano-capsules, nano-pincettes, AFM/STM tips, single electron transistors, gas sensors, fine parts for medical or engineering use, highly functional composites and the like.
When produced by conventional methods, a mixture of nanotubes of different properties is obtained; particularly, a mixture of semiconducting and metallic nanotubes is formed. In an attempt to separate nanotubes by their properties from the mixture of nanotubes having different properties, many studies have been conducted. However, no study has provided a practical method for the selective separation of semiconducting, or non-metallic nanotubes, and metallic nanotubes on an industrial scale.
Although R. Krupke suggested a method for separating metallic nanotubes from a mixture of nanotubes having various properties using, for example, dielectrophoresis, there have not been any methods suggested, so far, which successfully separate non-metallic, or semiconducting, nanotubes from the mixture of nanotubes.
In order to collect and use the separated nanotubes in the manufacturing of various devices, the nanotube mixture is treated, separated and collected in a powdered form.